Why is "Smooth" shading slower than "Flat" shading in the viewport?
Since I don't know how many polys you are talking about, and what hardware you are using, there is a general answer that smooth shading is always slower than flat.  Flat shading uses one normal per face to calculate the color of the entire face.  Smooth shading interpolates between the normals of each vertex to render a smoothly curved face. Since a face is at least 3 vertices (i.e. a triangle) then smooth shading will always be at least 3x as slow as flat.  

In most cases, even 3-4x as slow is a matter of milliseconds so you don't really notice, but on older hardware with a ton of faces you certainly could.